Micro-objects, such as biological cells, can be processed in microfluidic apparatuses. To facilitate such processing, droplets containing micro-objects or reagents used to process the micro-objects must be generated in a reliable and consistent manner. Present solutions for droplet generation are limited by the need for expensive, highly specialized surfactants. In addition, the droplets are typically produced continuously, at a high rate that complicates the subsequent movement and use of the droplets. Consequently, a need exists for improved methods of droplet generation that avoid the use of expensive surfactants and allow flexibility with regard to the timing and rate of droplet production.